An antenna or an antenna array is an essential part of many electronic systems that require independent and/or simultaneous transmission and receipt of two orthogonal polarizations. Typically, square planar patch antennas are used in modern telecom or radar systems due to their simplicity, good radio frequency (RF) performance, ease of manufacturing and low cost.
However, when two orthogonal, symmetrically placed ports (unbalanced excitation) are added to such an antenna, or any other antenna capable of sustaining two orthogonal polarizations, the resulting antenna cross-pol attenuation appears degraded to values that are insufficient for most applications (e.g. 20 dB or less). This degradation seems to occur regardless of the approach used to add the two orthogonal ports.
One explanation for this degradation is the presence of higher modes which are impairing the cross-pol attenuation and/or the unbalanced (e.g. different) port impedances, where the unbalance is caused by limited port isolation. It is generally believed that this phenomenon is built-in and cannot be alleviated. As a result, a number of design solutions have been devised to mitigate the degradation. One example design is “balanced” excitation, in which the patch is fed by four symmetrical ports, fed in pairs with opposite (0, −180°) phase.
However, many designs devised to mitigate the degradation come with severe limitations. For instance, the balanced approach adds significant signal loss due to the complexity of the feed network, degrading the overall antenna efficiency to intolerable levels, is sensitive to typical manufacturing errors and adds a frequency dependent phase run-out which further degrades the cross-pol. The 2×2 subarray approach having two-mirrored element pairs (as described in U.S. Pat. No. 6,147,648) does not allow scanning and forbids the synthesis of an arbitrary radiation characteristic or an adaptive antenna.
Other antenna approaches are based on frequency scanning which automatically means that such an antenna exhibits frequency squint which for frequency allocation reasons and/or frequency hopping reasons may be unacceptable.
Modern telecommunication and/or radar systems require an array antenna incorporating a patch design that meet the following requirements over the full system bandwidth:                Excellent effective radiation efficiency, comparable with a paraboloidal reflector        Orthogonal dual linear polarization and circular polarization left/right capability        Good cross-pol isolation in both linear and circular polarizations        Allows the synthesis of any radiation characteristic (cosecant2, etc.)        High peak power capability        No frequency induced squint        No matching network required (no added loss)        
All the above requirements are necessary to replicate the performance of the presently available antennas used in the current systems and to maintain system capabilities. If the array antenna in question is to be applied in the latest system designs (high performance weather radar, demanding space applications, etc.) additional requirements should also be met over the full system bandwidth:                Electronic scanning ability        Very high cross-pol attenuation (35-40 dB) in the full system bandwidth (linear and/or CP)        The cross-pol attenuation is maintained during scanning in principal planes        No phase run-out of the orthogonal ports over the system bandwidth        Simple patch design, low cost, tolerant to usual manufacturing accuracy        
Accordingly, there is a need for improved orthogonal polarization antennas, and in particular, for improved square patch antennas capable of offering broadband high cross-pol attenuation (linear V/H and/or left-right CP) for use in telecommunications, space-borne applications and radar systems, that meet the above requirements.